Alternanthera philoxeroides callus,cultured by tissue culture technique,was exposed to gradient concentration of Zn²⁺ (0,0.2,0.4,0.8,1.6 mmol/L) for 7 d.Oxidative stress and changes in polyamines metabolism caused by excessive zinc were investigated.Results showed that,with the increasing of Zn²⁺ concentration: (1) the rate of O₂^-· generation and thiobarbituric acid reacting substances content was enhanced remarkably,and the superoxide dismutase activity and the chlorophyll-a content were decreased singnificantly;(2) the activities of arginine decarboxylase and ornithine decarboxylase were increased gradually, while the activities of polyamine oxidases and diamine oxidases were increased at first and then decreased at higher concentrations;(3) the contents of total-putrescine and total-spermine was decreased at first and then increased,and the contents of total-spermidine only showed an remarkable increase at 1.6 mmol/L Zn²⁺ treatment;(4) the free-putrescine content changed in a similar pattern as its total counterpart,the free-spermidine content showed a decline at 0.8 mmol/L and 1.6 mmol/L Zn²⁺ treatment,the free-spermine content had little fluctuation except a decrease at 0.2 mmol/L Zn²⁺;(5) the contents of perchloric acid soluble conjugated (PS-conjugated) putrescine and spermine declined at 0.2 mmol/L Zn²⁺ treatment and then increased,the PS-conjugated spermine content continued to rise;and (6) the contents of perchloric acid insoluble bound (PIS-bound) putrescine and spermine were descended gradually.The data presented here showed that excessive Zn²⁺ treatment can cause evident oxidative stress and alter the polyamines metabolism balance.The conversion from PIS-bound polyamines to PS-conjugated polyamines was one of the key characters of polyamines metabolism alternation in A.philoxeroides callus under excessive Zn²⁺ treatment.The accumulation in the total polyamines characterized by the PS-conjugated polyamines played an important role in A.philoxeroides cal-lus in response to excessive Zn²⁺ treatment.